Modeling of the influence of deep majority carrier traps on photo-induced current transients in Cu(In, Ga)Se2 layers

In this contribution we aim at modeling of an unusual PITS response of deep majority carrier traps experimentally detected in conductive p-type CIGS layers. We numerically solve the set of kinetic occupation equations, and we distinguish two situations in which such defects can contribute to the PITS signal: (i) Fermi level is pinned by a very deep donor of high concentration slightly above the acceptor level, and (ii) metastable defects with a negative-U energy are involved. In the latter case activation energies extracted from Arrhenius plots do not represent the energetic location of the levels within a bandgap but are related to the structural barriers for a defect conversion. In addition, we note that the contribution of hole capture by deep acceptor states can explain experimental PITS signals, observed in highly conductive layers, characterized by a negative temperature activation. Then, the activation energy strictly reflects the temperature dependence of the Fermi level position with respect to the valence band maximum.